KrF excimer lasers are currently becoming the workhorse light source for the integrated circuit lithography industry. A typical prior-art KrF excimer laser in the production used integrated circuits is depicted in FIG. 1 and FIG. 2. A cross section of the laser chamber of this prior art laser is shown in FIG. 3. A pulse power module 2 powered by high voltage power supply 3 provides electrical pulses to electrodes 6 located in a discharge chamber 8. The electrodes are about 28 inches long and are spaced apart about 3/5 inch. Typical lithography lasers operated at a high pulse rate of about 1000 Hz. For this reason it is necessary to circulate a laser gas (about 0.1% fluorine, 1.3% krypton and the rest neon which functions as a buffer gas) through the space between the electrodes. This is done with tangential blower 10 located in the laser discharge chamber. The laser gases are cooled with a heat exchanger 11 also located in the chamber and a cold plate 13 mounted on the outside of the chamber. Cooling water for cold plate 13 and heat exchanger 11 enters at water inlet 40 and exits at water outlet 42 as shown in FIG. 3. The natural bandwidth of the KrF laser is narrowed by line narrowing module 18. Commercial excimer laser systems are typically comprised of several modules that may be replaced quickly without disturbing the rest of the system. Principal modules are shown in FIG. 2 and include:
Laser Chamber 8, PA0 Pulse Power Module 2, PA0 Output coupler 16, PA0 Line Narrowing Module 18 PA0 Wavemeter 20 PA0 Computer Control Unit 22
These modules are designed for quick replacement as individual units to minimize down time to the laser when maintenance is performed. Electrodes 6 consist of cathode 6A and anode 6B. Anode 6B is supported in this prior art embodiment by anode support bar 44 which is about 28 inches long and is shown in cross section in FIG. 3. Flow is clockwise in this view. One corner and one edge of anode support bar 44 serves as a guide vane to force air from blower 10 to flow between electrodes 6A and 6B. Other guide vanes in this prior art laser are shown at 46, 48 and 50. Perforated current return plate 52 helps ground anode 6B to chamber 8. The plate is perforated with large holes (not shown in FIG. 3) located in the laser gas flow path so that the plate does not substantially affect the gas flow. Electrode discharge capacitors 54 are charged prior to each pulse by pulse power module 2. During the voltage buildup on capacitor 54 a high electric field is created by two preionizers 56 which produce an ion field between electrodes 6A and 6B and as the charge on capacitors reach about 16,000 volts, a discharge across the electrode is generated producing the excimer laser pulse. Following each pulse, the gas flow created by blower 10 is sufficient to provide fresh laser gas between the electrodes in time for the next pulse occurring 1.0 milliseconds later.
The discharge chamber is operated at a pressure of about three atmospheres. These lasers operate in a pulse mode at high repetition rates such as 1000 Hz. The energy per pulse is about 10 mJ.
At wavelengths below 300 nm there are few optical materials available for building the stepper lens used for chip lithography. The most common material is fused silica. An all fused silica stepper lens will have no chromatic correction capability. The KrF excimer laser has a natural bandwidth of approximately 300 pm (full width half maximum). For a refractive lens system (with NA&gt;0.5)--either a stepper or a scanner--this bandwidth needs to be reduced to below 1 pm to avoid chromatic aberrations. Prior art commercially-available laser systems can provide KrF laser beams at a nominal wavelength of about 248 nm with a bandwidth of about 0.8 pm (0.0008 nm). Wavelength stability on the best commercial lasers is about 0.25 pm. With these parameters stepper makers can provide stepper equipment to provide integrated circuit resolutions of about 0.3 microns.
Electric discharge lasers such as excimer lasers require high voltage power supplies. A prior art typical simplified electric circuit for an excimer laser is shown in FIG. 4. The electric circuit includes a magnetic switch circuit and a power supply for the magnetic switch circuit. Blocks representing a 1 kV prior-art power supply for the laser are shown at 3 in FIG. 2 and FIG. 4. A more detailed description of the prior art power supply is shown in FIG. 5A. In the typical prior art laser system, the power supply 2 provides high voltage pulses of about 600 volts lasting about 0.2 milliseconds at frequencies such as 1000 Hz. The magnetic switch circuit shown in FIG. 4 compresses and amplifies these pulses to produce electrical discharges across the electrodes of the laser as shown in FIG. 4. These discharge pulses across the electrodes are typically about 16,000 volts with duration of about 70 ns.
Maintaining constant power supply output voltage when the laser is operating continuously at a specific repetition rate, such as 1000 Hz, is a challenge for laser suppliers. This task is made much more difficult when the laser is operated in a burst mode. A typical burst mode is one in which the laser is required to produce bursts of about 110 pulses at a rate of 1000 kHz during the bursts with the bursts being separated by a "dead time" of a fraction of a second to a few seconds. When operating in a continuous mode, the output voltage variation, to maintain relatively constant output pulse energy, is in the range of about 0.6% (about 3 to 3.5 volts). When operating in the burst mode, the variation during the first few pulses (up to about 40 pulses) is about 2.5% (about 12 to 15 volts) and precise control of pulse energy variation is not as good.
In a typical lithography excimer laser, a feedback control system measures the output laser energy of each pulse, determines the degree of deviation from a desired pulse energy, and then sends a signal to a controller to adjust the power supply voltage so that energy of subsequent pulses are closer to desired energy. In prior art systems, this feedback signal is an analog signal and it is subject to noise produced by the laser environment. This noise can result in erroneous power supply voltages being provided and can in turn result in increased variation in the output laser pulse energy.
These excimer lasers are typically required to operate continuously 24 hours per day, 7 days per week for several months, with only short outages for scheduled maintenance. One problem experienced with these prior-art lasers has been excessive wear and occasional failure of blower bearings.
The prior-art wavemeter module shown in FIG. 2 is described in FIG. 6. The wavemeter utilizes a grating for coarse measurement of wavelength and an etalon for fine wavelength measurement and contains an iron vapor absorption cell to provide an absolute calibration for the wavemeter. This prior art device focuses the coarse signal from the grating on a linear photo diode array in the center of a set of fringe rings produced by the etalon. The center fringes produced by the etalon are blocked to permit the photo diode array to detect the coarse grating signal. The prior-art wavemeter cannot meet desired accuracy requirements for wavelength measurements.
Prior-art lasers such as the one discussed above are very reliable, producing billions of pulses before the need for major maintenance, but integrated circuit fabricators are insisting on even better performance and reliability. Therefore, a need exists for a reliable, production quality excimer laser system, capable of long-term factory operation and having wavelength stability of less than 0.2 pm and a bandwidth of less than 0.6 pm.